1) Field of the Invention
The present invention relates to an alkaliresistant glass for forming glass fibers used to reinforce cementitious products, and more particularly to an alkaliresistant glass with a low devitrification temperature and superior fiberizing characteristics for forming glass fibers.
2) Description of the Prior Art
Glass fibers have not only so large a strength but also so high a modulus of elasticity that they are widely used to reinforce cementitious products. However cement is highly alkaline. Therefore, when glass fibers are mixed into cement the glass fibers are gradually attacked by the cement and break down so that their reinforcing effect is drastically reduced and the strength of the product itself is also lessened. For this reason there is a strong need for an alkali-resistant glass fiber for such an application.
Conventionally, it is known that the addition of ZrO.sub.2 to glass is effective in improving the resistance of glass to alkali. As the amount of ZrO.sub.2 in the glass composition is increased, the alkali resistance of the glass fiber obtained from such a composition also increases. However, as the ZrO.sub.2 content is increased, the fiberizing characteristics of the glass drastically deteriorate. Specifically, as the ZrO.sub.2 content is increased, the viscosity of the glass also increases, and the fiberizing temperature of the glass (the temperature at which the viscosity of the glass reaches 10.sup.3 poise - hereinafter referred to as T.sub.F) rises so that severe damage occurs to the platinum bushing used in fiberizing. In addition, in company with an increase in the ZrO.sub.2 content, the devitrification temperature of the glass (the temperature at which crystals first appear when molten glass is slowly cooled - hereinafter referred to as T.sub.L) rises, and there is a tendency for devitrification to occur in the liquid glass during melting and fiberizing. For this reason, a devitrified material becomes mixed into the liquid glass during fiberizing so that breaks in the fiber occur and fiberizing becomes very difficult. Accordingly, conventionally proposed alkali-resistant glass fibers containing ZrO.sub.2 are restricted to a relatively low ZrO.sub.2 content, or attempts are made to improve the fiberizing characterlstics by mixing in other components.
For example, Japanese Laid Patent Publication Nos. 53-18042 and 57-16939 disclose glass compositions with relatively low ZrO.sub.2 contents of less than 11 wt. % and less than 15 wt. % respectively. However, as previously outlined, because the resistance of glass to alkali and the ZrO.sub.2 content of the glass are interdependent, when the ZrO.sub.2 content is held to a low value the alkali resistance of the glass is insufficient for use in practice.
In addition, a composition wherein other components are mixed into the glass in addition to an increase in the ZrO.sub.2 content is disclosed in Japanese Patent Publication No. 60-54248 in which CaO and BaO are blended into the mixture to control devitrification. However, an example disclosed in this patent is seen to have a T.sub.F of 1300.degree. C. or higher, which is high enough to cause concern about the possible deterioration of the platinum bushing or about difficulties in fiberizing the glass because the difference between the T.sub.L and the T.sub.F, on which the fiberizing characteristics are based, is 100.degree. C. or less. Specifically, the results of experiments conducted by the inventors of the present invention have indicated that it is preferable that the T.sub.F be lower than 1300.degree. C. to prevent deterioration of the platinum bushing. In addition, even if the T.sub.F is set at a temperature which the platinum bushing can withstand, in the immediate vicinity of the molten glass, for example, part of the molten glass halfway through a feeder which conducts it to the platinum bushing from the melting furnace drops to a temperature considerably lower than the T.sub.F. Accordingly, if the T.sub.L is not considerably lower than the T.sub.F, devitrification occurs causing fiber breakage. For this reason, it will be essential that T.sub.F -T.sub.L be 150.degree. C. or greater for a stable industrial fiberizing operation.
Examples for which T.sub.F -T.sub.L is 150.degree. C. or greater are described in Japanese Patent Publication No. 49-40126, but all the examples which satisfy this condition have a ZrO.sub.2 content of less than 18 wt. %. Therefore the alkali resistance of the glass is unsatisfactory.
The addition of rare earth metal oxides such as CeO.sub.2 and La.sub.2 O.sub.3 to the glass is not effective for improvement of the quality of the glass for forming glass fiber because such rare earth metal oxides devitrify the glass and are costly.